Recently, electric and electronic apparatuses have had smaller sizes and have been used in a higher frequency. In an inductance component that is one of important electronic components used in such apparatuses, high performance magnetic materials capable of achieving magnetic elements having a small size and high efficiency have been required. Thus, for a choke coil or the like used in a high frequency region, a ferrite core or a dust core is used as the magnetic material. Among them, the ferrite core formed of relatively low-price metallic oxide has a low saturated magnetic flux density. The dust core produced by molding a metal magnetic powder has a remarkably high saturated magnetic flux density as compared with that of the ferrite core. However, the dust core has a large core loss. The core loss includes a hysteresis loss and an eddy current loss. The eddy current loss is increased in proportion to the square of the frequency and the square of the size of eddy-current flow. In order to suppress generation of the eddy current, it is known to cover the surface of the metal magnetic powder with electric insulating resin or the like. On the other hand, the hysteresis loss is increased when the dust core is molded at a pressure of not less than several ton/cm2. This is because distortion of the dust core as the magnetic material is increased and, at the same time, the relative magnetic permeability is reduced. In order to prevent the increase in the hysteresis loss, as described in, for example, Patent Literature 1, it is known that heat annealing treatment is carried out after the dust core is molded.
In general, as a soft magnetic alloy powder contains more iron (Fe) components, it has a higher saturated magnetic flux density and therefore is advantageous in a direct superposition property. On the other hand, as the more Fe components are contained, rust is generated at a high temperature and a high humidity. When a magnetic element is mounted on a circuit board and the rust drops on the board, circuit operation failure may occur.
The surface of the metal magnetic powder is covered with an organic electric insulating material, an inorganic electric insulating material, or the like. However, when a molded product is released from a mold at the time of press-molding of the dust core, the insulating material on the side surface of the molded product which is brought into contact with a mold surface is easily peeled off. Therefore, in the final product, rust is generated remarkably in a portion on which the insulating material is peeled off. Furthermore, when the molded product has a profile shape and a larger size, for example, when the molded product has an E-profile shape and a size of not less than 15 mm2, when the molded product is released from a mold, a pulling pressure is partially concentrated for a long time as compared with a small molded product. Consequently, an insulating layer on the surface of the metal magnetic powder on the side surface of the molded product that is brought into contact with the mold is easily peeled off, and rust is easily generated.
For such problems, for example, Patent Literature 2 describes addition of Cr having a corrosion resistance effect as the magnetic alloy. However, in a case of a low-loss magnetic material that is subjected to heat treatment of not lower than 600° C., the magnetic property is remarkably lowered although the cause thereof is not clear.
Thus, it is difficult to achieve both corrosion resistance and soft magnetic property. Therefore, measures such as covering a core portion of a final product with protective coating such as resin, or filling the core portion in a protective case, or the like, are taken. However, such measures are not only disadvantageous in terms of reduction in size and cost but also insufficient in the reliability.